The most extensive survey of pH levels in the Pacific Ocean confirms what spot measurements have suggested: From Hawaii to Alaska, the upper reaches of the sea are becoming more acidic in concert with rising carbon dioxide in the atmosphere.

“If you see these changes across an entire ocean basin, you can be assured it’s happening on a global scale in other ocean basins around the world,” said Robert Byrne, a marine chemist at the University of South Florida and lead author of an upcoming paper in Geophysical Research Letters.

Ocean acidification is a threat to shelled creatures and other marine life, and is a leading suspect in the ongoing crash of Pacific oyster populations in Washington.

Byrne collaborated with Seattle scientists on the survey, which was 15 years in the making. The team first measured acidity along the 2,800-mile sweep of ocean between Oahu and Kodiak in 1991. They returned in 2006 aboard the University of Washington research vessel Thomas G. Thompson, working around the clock in shifts to collect and analyze nearly 1,500 water samples over two months.

It’s the first time measurements have been taken across such a wide area, said co-author Richard Feely, of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle.

“The fact that we saw this very significant change over the last 15 years is a reminder of how mankind is affecting the oceans at an ever-increasing rate,” Feely said.

In addition to contributing to a global greenhouse effect, some of the carbon dioxide from cars, factories and power plants dissolves in the ocean, creating the same carbonic acid that gives soda pop its tang. The process makes seawater slightly more acidic, and also gobbles up carbonate, a basic building block of seashells.

The result can be an environment where shells dissolve, destroying plankton, marine snails and other small creatures that sustain the rest of the marine food web. Acidified water also can kill fish eggs and larvae.

Byrne and his colleagues developed a more precise way to measure pH, using a dye that turns from purple to bright yellow as acidity increases. On board the ship, they used instruments called spectrophotometers to measure the color change and nail pH levels 10 times more accurately than possible before.

Debby Ianson, an ocean climate modeler for Canada’s Institute of Ocean Sciences who was not involved in the project, said the approach is a good one. “We need studies like this,” she wrote in an e-mail.

As expected, the researchers found acidification was strongest in the top layer of water, closest to the atmosphere. Normal seawater is slightly alkaline, with a pH value of about 8. Over the past 15 years, average pH levels in the top 300 feet of the ocean dropped 0.026 pH units. That sounds tiny, but is equivalent to a 6 percent jump in acidity, Byrne said.

Working a thousand or more miles off the West Coast, the scientists took samples down to the ocean floor. They found no change yet in acidity at the deepest levels. But as carbon-dioxide levels in the atmosphere continue to climb, natural mixing eventually will raise acidity throughout the water column, Byrne said.

Analysis of air bubbles trapped in Antarctic ice cores show that atmospheric carbon dioxide already is higher than anytime in the last 800,000 years, and the same is almost certainly true of ocean acidity, Byrne pointed out. Since the start of the Industrial Age, the scientists calculate that the acidity of the world’s oceans has increased by 25 to 30 percent.

Under a business-as-usual scenario, Feely said, ocean acidity could triple by the end of the century.

Ianson cautions that extrapolating future rates of acidification is difficult, because the ocean and atmosphere are so complex.

Still, Byrne said, the trend is clear.

“We’re seeing unprecedented rates of change in the atmosphere, and we’re going to see changes in seawater in lock-step with that,” he said.